Magnetoresistive random-access memory (“MRAM”) is a non-volatile memory technology that stores data through magnetic storage elements. These elements are two ferromagnetic plates or electrodes that can hold magnetization and are separated by a non-magnetic material, such as a non-magnetic metal or insulator. In general, one of the plates has its magnetization pinned (i.e., a “reference layer”), meaning that this layer has a higher coercivity than the other layer and requires a larger magnetic field or spin-polarized current to change the orientation of its magnetization. The second plate is typically referred to as the free layer and its magnetization direction can be changed by a smaller magnetic field or spin-polarized current relative to the reference layer.
MRAM devices store information by changing the orientation of the magnetization of the free layer. In particular, based on whether the free layer is in a parallel or anti-parallel alignment relative to the reference layer, either a “1” or a “0” can be stored in each MRAM cell. Due to the spin-polarized electron tunneling effect, the electrical resistance of the cell changes due to the orientation of the magnetization of the two layers. The cell's resistance will be different for the parallel and anti-parallel states and thus the cell's resistance can be used to distinguish between a “1” and a “0.” One important feature of MRAM devices is that they are non-volatile memory devices, since they maintain the information even when the power is off. The two plates can be sub-micron in lateral size and the magnetization direction can still be stable with respect to thermal fluctuations.
FIG. 1A illustrates a magnetic tunnel junction (“MTJ”) 130 for a conventional MRAM device. The MTJ 130 includes reference layer 132, which is a magnetic layer, a non-magnetic tunneling barrier layer 134, which generally is an insulator at large layer thickness but at small layer thickness admits the flow of an appreciable quantum mechanical tunnel current, and a free layer 136, which is also a magnetic layer. The magnetization direction of the magnetic layers of the MTJ 130 can be in the planes of those layers or perpendicular to the planes of those layers. As shown in FIG. 1A, the magnetic reference layer 132 has a magnetization direction perpendicular to its plane. Also as seen in FIG. 1A, the free layer 136 also has a magnetization direction perpendicular to its plane, but its direction can vary by 180 degrees. Because the magnetization direction of the magnetic layers of the MTJ 130 is perpendicular to the planes of those layers, the MTJ 130 shown in FIG. 1A is known as a perpendicular MTJ (“pMTJ”).
As shown in FIG. 1A, electrical contact layers 170, 172 can optionally be used to provide electrical contact to the MTJ 130. When the magnetization of the free layer 136 is oriented in a direction parallel to the magnetization direction of the reference layer 132, electrons will be more likely to tunnel across the tunneling barrier layer 134, and thus resistance across the MTJ 130 will be lower. Alternatively, when the magnetization of the free layer 136 is oriented in a direction that is anti-parallel to the magnetization direction of the reference layer 132, electrons will be less likely to tunnel across the tunneling barrier layer 134, making the resistance across the MTJ 130 significantly higher. It is these different resistances that can be used to distinguish and store a digital “1” or “0” bit.
The MTJ 130 may also form part of a larger MTJ stack 100, as shown in FIG. 1B, which may include a number of other optional layers that can be used to facilitate operation of the MTJ. As described in connection with FIG. 1A, the MTJ stack 100 of FIG. 1B may include electrical contact layers 170, 172 for providing electrical contact across the MTJ stack 100, including the MTJ 130. The MTJ 130 may be disposed above an antiferromagnetic layer or a synthetic antiferromagnetic (“SAF”) structure 120, which may include multiple layers as shown in FIG. 1B. For example, as shown in FIG. 1B, the SAF structure 120 may include two or more thin magnetic layers, including a lower “SAF1” layer 122 and an upper “SAF2” layer 126 having opposite or anti-parallel magnetization directions separated by an antiferromagnetic coupling layer 124 or spacer layer that is not magnetic. The SAF structure 120 also may be formed over a seed layer 110, as shown in FIG. 1B, and over a substrate (not shown). Note that as used herein, terms such as “lower,” “upper,” “top,” “bottom,” and the like are provided for convenience in explaining the various embodiments, and are not limiting in any way.
Spin transfer torque or spin transfer switching, may be used in connection with an MTJ 130. In such a configuration, a filter layer 150 may be used to alter the spin of electrons passing through the MTJ 130. For example, the filter layer may be a polarizer layer designed to further align the spin of electrons (i.e., to further “polarize” the electrons) passing through the MTJ 130 beyond the alignment already provided by the reference layer 132. U.S. patent application Ser. No. 14/814,036, filed by Pinarbasi et al., and assigned to the assignee of this patent document describes using a polarizer layer. The disclosure of U.S. patent application Ser. No. 14/814,036 is incorporated herein by reference in its entirety. The spin-aligned or “polarized” electrons are used to change the magnetization orientation of the free layer 136 in the MTJ 130. In general, electrons possess a spin, a quantized number of angular momentum intrinsic to the electron. An electrical current is generally unpolarized, i.e., it consists of 50% spin up and 50% spin down electrons. Passing a current though a magnetic layer, like the filter layer 150 or the reference layer 132, polarizes electrons with the spin orientation corresponding to the magnetization direction of the magnetic layer, thus producing a spin-polarized current. If a spin-polarized current is passed to the magnetic region of a free layer 136 of the MTJ 130, the electrons will transfer a portion of their spin-angular momentum to the free layer to produce a torque on the magnetization of the free layer. Thus, this spin transfer torque can switch the magnetization of the free layer, and can be used to write either a “1” or a “0” based on whether the free layer 136 is in the parallel or anti-parallel states relative to the reference layer.
As shown in FIG. 1B, the filter layer 150 and a filter coupling layer 140 are disposed above the free layer 136 of the MTJ 130. The filter layer 150 is physically separated from the free layer 136 and is coupled to the free layer 136 by the coupling layer 140. The filter layer 150 can be used to polarize a current of electrons (“spin-aligned electrons”) applied to MTJ structure 100 in the manner described above.
The filter layer 150 is a precessional spin current (“PSC”) magnetic layer, an example of which is described in detail in U.S. patent application Ser. No. 14/814,036, filed by Pinarbasi et al., and assigned to the assignee of this patent document. Additionally, a precessional spin current structure for MRAM is described in U.S. patent application Ser. Nos. 15/445,260 and 15/445,362, both of which are filed by Pinarbasi et al. and are assigned to the assignee of this patent document. The disclosures of U.S. patent application Ser. Nos. 15/445,260 and 15/445,362 are both incorporated herein by reference in their entireties.
Switching speed, switching current, and thermal barrier height are all parameters that affect the overall performance of MRAM devices. In such devices, it is desirable to maximize switching speed while minimizing switching current and thermal barrier height. However, a selected value for one of those parameters may limit the possible values for others of those parameters. Thus, tradeoffs between selected values must be made in order to maintain optimal performance. In conventional pMTJ devices, the tradeoff between switching speed, switching current, and thermal barrier height, while important, has been difficult to optimize. Moreover, in conventional pMTJ devices, the filter layer (e.g., precessional spin current layer) has a size and shape that are the same as those of the free layer. For example, in conventional pMTJ devices, the filter layer has a diameter that is the same as a diameter of the free layer. Furthermore, in conventional pMTJ devices, the filter layer and the free layer are formed coaxially with one another. Also, in conventional pMTJ devices, the filter layer has a moment density that is uniform throughout the layer.